This is how it looks for me when I take a extension with a alpha channel over a simple BG and turn Cuda on then turn Cuda off.
The image above is with GPU acceleration turned on.
This image below this sentence is with software on.
In a way it appears simliar to your findings since when my GPU accleration is turned on you can see a white boarder creeping up from the bottom and top but with it off you don't see it.
However when I turn the blur up higher and try it again I get these results.
GPU On in the image directly below.
GPU off in the image below this sentence.
I'm using the exact same blur settings in both images... In the top image you can see that white is taking over the picture.
This has GPU on in the image below this sentence.
This is Software only on in the pic below this sentence.
Not really sure if this confirms anything for you or not though. Just figured I'd post it cause the issue seemed intresting to me. Anyways though here were my findings.
Yeah it does appear to pull the edge inwards. Almost just like feathering